The present invention relates to a method of forming a three-dimensional network porous metallic structure having a continuous internal cavity, characterized by an extremely large specific surface area and porosity and, accordingly, minimized pressure drop of a fluid passing through the structure.
The three-dimensional network porous metallic structure of the invention has, owing to its advantageous characteristics, various extended industrial uses such as the materials for fins of heat exchangers, liquid-liquid or liquid-gass mixer or separator, sound absorbing or insulating material, damping material, catalysts, flame extinguishing materials, wick materials for heat pipes, ornamental materials, architectural materials and so forth.
Conventionally, various methods have been proposed and practically carried out for forming porous metallic structures.
These conventional methods are, for example:
(a) to blow a gas into a molten metal to allow the metal to foam;
(b) to make pulverized or granulated metal to foam;
(c) to insert a substance which evolves a gas when heated into a molten metal;
(d) to heat to high temperature an admixture of a metal and a substance which evolves a gas when heated;
(e) to press-feed a molten metal into a mold cavity charged with sodium chloride particles;
(f) to allow a molten metal to flow out through a narrow nozzle;
(g) to obtain the porous metal by sintering; and so forth.
All of above mentioned methods, except the method (g), have only limited use, because the internal cavities in the porous metal product are not made continuous or, even if made continuous, they communicate one another only through extremely narrow, coarctate parts, resulting in various shortcomings such as unacceptably large flow resistance by which the gas flowing therethrough is encountered. At the same time, the porosity is as small as 50% or so, and the specific surface area is disadvantageously small.
The method (g) relying upon the sintering is also disadvantageous in that it cannot be used for forming such a thick structure as having a thickness exceeding about 15 mm. At the same time, this method is quite time-consuming and invonveniently raises the cost of manufacture. Further, this method can be used effectively only with some specific metals such as alloys of Nickel (Ni), Copper (Cu) and Iron (Fe). Thus, this method cannot be applied to the formation of porous body with Aluminum (Al) and its alloy.